As shown in FIG. 8, a wafer transfer system A′ constituting a semiconductor fabricating system (not shown) is designed to take wafers U, one by one, out of a wafer carrier C, which is provided on the top face of a load port unit L, to transfer the wafers U to a wafer processing system E and to transfer the wafers U, which have been processed by the wafer processing system E, to the wafer carrier C again. At this time, if dust adheres to the wafers U, the wafers U can be defective. In order to prevent this, a clean air supply system 4 is provided in the upper portion of the wafer transfer system A′ for always supplying clean air K from the unit 4 to the wafers U. This clean air K is blown from top to bottom of a wafer transfer robot R constituting the wafer transfer system A′. Dust existing in the wafer transfer system A′ is carried toward the bottom by the flow of the clean air K, and exhausted by an exhaust fan 5 which is provided on the bottom portion 1c of a system body 1. Thus, dust is prevented from adhering to the wafers U.
In a conventional wafer transfer system, a ball screw and a control motor are used for linearly reciprocating a wafer transfer robot. Since the distance of the movement of the wafer transfer robot is not long in the case of the ball screw, a wafer transfer system for linearly reciprocating the wafer transfer robot by means of a linear motor has been developed.
Referring to FIG. 8, a wafer transfer system A′ using a linear motor M will be described below. In a case where the linear motor M is used, the secondary side 11 of the motor M is usually mounted on the bottom portion 1c of the system body 1. However, in this case, the portion of the exhaust fan 5 is covered with the secondary side 11 of the linear motor M, so that there is a problem in that exhaust efficiency deteriorates. In addition, since the wide area of the secondary side 11 of the linear motor M faces upwards, dust is easy to be deposited on the top face of the secondary side 11, and this dust is sometimes carried by the flow of the clean air K to float. In such a case, dust is easy to adhere to the wafers U, so that the rate of occurrence of defective wafers U rises.
The load port unit L is mounted on the outside of the front wall of the system body 1, and the wafer carrier C is provided on the top face of the load port unit L. By mounting the linear motor M on the bottom portion 1c of the system body 1, the distance between the wafer transfer robot R, which is provided on the linear motor M, and the wafer carrier C is long, and a large number of members exist between the wafer carrier and the wafer transfer robot R. As a result, production errors in the members accumulate, so that there is a problem in that the wafer transfer robot R can not precisely position the wafers in the wafer carrier C.
In a system of this type, there are some cases where the wafer transfer robot R is emergency-stopped in view of safety work during the operator's maintenance and inspection work. In the case of the conventional wafer transfer system A′, it is possible to emergency-stop the wafer transfer robot R with a relatively simple construction by incorporating an electromagnetic brake or the like in the control motor for driving the ball screw. However, in the case of the linear motor M, it is very difficult to incorporate the above described electromagnetic brake or the like.
Means for stopping the wafer transfer robot R using the linear motor M will be described below. In the control circuit for the linear motor M, a regenerative braking is incorporated. By operating the regenerative braking, the wafer transfer robot R can be stopped at a predetermined position. In this control circuit, an uninterruptive power supply is also incorporated. Therefore, if the wafer transfer robot R intends to travel due to inertia when the feeding of power is interrupted by an interruption of the power supply or the like, the feeding of power can continue for a set-up time which is set in the uninterruptive power supply. Then, the regenerative braking is operated within the set-up time to stop the wafer transfer robot R. However, there are some cases where the wafer transfer robot travels due to inertia so as not to be stopped even if the set-up time of the uninterruptive power supply elapses. There is the possibility that this malfunction may quite similarly occur even if an emergency stop switch for interrupting the feeding of power to the wafer transfer system A is operated in an emergency.